Display devices with a touch sensor, in which a touch sensor (also called a “touch panel”) is provided on the front face (observer side) of a display, have conventionally been used in various applications. A touch sensor is an input device that enables operation instructions to be given and data to be input by detecting the position of a place where a finger, pen, or the like came into contact. Examples of known systems for position detection include a capacitive coupling system, a resistive film system, an infrared system, an ultrasonic system, and an electromagnetic induction/coupling system.
When a touch sensor that is integrated with a display device is used, the problem of a reduction in the precision of position detection by the touch sensor occurs due to the touch sensor being subjected to noise from the display device. For example, when a liquid crystal panel is used as the display device, induced voltage is generated in the conductive films for position detection of the touch sensor due to the common voltage that is applied to the counter electrode of the liquid crystal panel. This induced voltage is a cause for noise.
A configuration for removing this kind of noise is disclosed in JP 2006-146895A, for example. The display device with a touch sensor disclosed in this patent literature includes a strobe signal generation circuit and a noise-removed current signal generation circuit. The strobe signal generation circuit generates a strobe signal that is synchronized with the cycle of the polarity inversion of the common voltage that is supplied to the counter electrode. The noise-removed current signal generation circuit generates a noise-removed current signal by, based on the strobe signal, removing a predetermined portion from the current flowing from the terminal that is connected to the touch sensor unit.
According to this conventional configuration, a strobe signal is used to remove noise that appears in the output current of the conductive films for position detection due to the cyclical polarity inversion of the common voltage. This improves the SN ratio of the touch sensor output and improves the precision of position detection.
However, the above-described conventional configuration has a complicated structure due to requiring dedicated circuits for noise removal, namely the strobe signal generation circuit and the noise-removed current signal generation circuit.